Tanner Springs Park in Portland, Oregon is not a restored wetlands, but it mimics one. As a bold gesture of the local ecology in a post-industrial Pearl District, the park’s central feature is a stormwater management waterscape expressing the history of Tanner Creek. It’s hard to believe but Tanner Creek once flowed directly into Couch Lake, the very site now known as Pearl District.

The stormwater management method is pretty straightforward: all of the rainwater that falls on the park drains to the park’s “wetland” section and lower pond. The untreated water flows into an underground cistern, and then through an ultraviolet vault. From there the treated water is pumped up to an artificial spring, making its way down the artificial streams, and back into the “wetland.”

Wanting to learn more about the technical processes of this simulated wetland, I turned to the generic water treatment diagram at the park’s information kiosk, but it proved to provide little insight. I also went to the German landscape architect’s website, Atelier Dreiseitl, hoping to gain some technical insight into the functions, but again to no success.

My primary question was where Tanner Springs Park was receiving its supplemental water to sustain the park’s wetland feature between rainfall? I understand that some of the water is from stormwater, since 100% of water that lands on the site is circulated into the wetland system. But what about during days when it has not rained and water has evaporated? Yes, Portland gets a lot of rain – around 40 inches per year on average. But during the summers, it’s hot and dry. I suspected the park had another water source that sustains the park.

After searching on the internet for more information, I came across the following article from ASLA which noted:

“Rainwater will have to be occasionally topped off with city water to compensate for natural evaporation and water splash as well as Portland’s long dry summers; nevertheless, the requirement for city water will be substantially reduced. During heavy rain periods, an overflow system is tied to the city’s stormwater sewers.”

So there was my answer. I just wish there was a better diagram that communicated the whole picture of Tanner Springs Park.

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